This invention relates to an encoder-transmitter useful in monitoring the height of liquid in a liquid storage tank. More particularly, the invention relates to such an encoder-transmitter which utilizes patterns of signals which are translatable into the absolute height of liquid over at least a portion of the total height of the tank.
The height of liquid in a storage tank is conventionally determined using a float-type mechanical gage. The float, which moves up and down in response to changes in liquid height, is attached to a gage shaft which rotates in response to the movement of the float. Thus, by calibrating the rotational position of the gage shaft with various heights of liquid, the liquid height at any given time can be determined by observing the rotational position of the gage shaft.
In many situations, liquid heights in a great many tanks, e.g., 50 to 100 or more tanks, need to be determined or monitored at the same time. Therefore, it is desirable to provide an encoder-transmitter for liquid height information from individual storage tank gages so that such information can be transmitted to a central receiving or monitoring station.
Various shaft position monitoring systems have been suggested in the prior art. Examples of such systems are disclosed in the following U.S. Pat. Nos. 4,137,451; 4,086,580; 4,037,219; 3,959,628; 3,875,406; 3,842,268; 3,808,431; 3,525,094; and 3,328,591.
Many of these prior art devices are not concerned with monitoring liquid heights in storage tanks. These previous transmitters have only a relatively limited degree of accuracy; require a relatively large amount of energy during use and/or may be unsafe, especially when used in conjunction with tanks used to store flammable liquids such as certain hydrocarbons. One additional problem with certain of the prior systems is the need for recalibration, e.g., between the gage and transmitter, each time the transmitter inadvertently lost track of the position of the gage shaft, e.g., during temporary power outages. This problem could involve substantial manpower requirements for recalibration. Also, since it may not be apparent that the transmitter has lost track of the position of the gage shaft, e.g., as the result of very short power outages, such systems may provide inaccurate liquid height information.